The present invention relates to an optical transmission apparatus and an optical transmission wavelength control method, used in wavelength division multiplexed (WDM) optical transmission systems. In particular, the invention relates to an optical transmission apparatus for multiple wavelengths, which can switch and transmit optical signals of a plurality of wavelengths, and an optical transmission wavelength control method.
With recent high information orientation, in optical communication, a significant increase in transmission capacity together with a reduction in transmission cost is required. In order to meet these requirements, up until now a transmission speed has mainly been increased. However, with an ultra high transmission speed, it has become technically difficult to bring such optical communication into practical use. Furthermore, an increase in cost has been resulted. Therefore, wavelength division multiplexed (WDM) techniques enabling of transmitting a plurality of optical signals of different wavelengths on a single optical fiber, are attracted attention.
In a WDM optical transmission system, in general, a plurality of optical signals with equal inter-wavelength spacing are transmitted via a single optical fiber. However, in the case of systems and the like for realizing 32 waves multiplexing conforming to ITU (International Telecommunication Union) recommendations for example, the inter-wavelength spacing becomes narrow at 0.8 nm. Therefore, suppression of inter-channel crosstalk has been attempted by providing a narrow-band optical filter in the optical multiplexing apparatus or the like to perform the limitation of band and attenuating wavelengths other than the necessary wavelengths.
However, if the wavelength of the optical signal output from the optical transmission apparatus and the like is deviated from the transmission central wavelength of the narrow-band optical filter, the optical output power is attenuated by the narrow-band optical filter, and thus reduced so that transmission quality is degraded. Therefore, since it becomes necessary to coincide the optical output wavelength of the optical transmission apparatus with the central wavelength of the narrow-band optical filter of the optical multiplexing apparatus, a wavelength control of the optical output is required to be performed with good accuracy.
In a wavelength control performed by a conventional optical transmission apparatus, for example, a part of light output from a laser diode (LD) is taken out, and input into a wavelength detection filter in which an optical filter with the transmissivity differing depending on wavelength is used. Light that has passed through the wavelength detection filter is received by a photodiode (PD), and a PD output current is converted into a voltage. Thereafter, this voltage is compared with a reference voltage set corresponding to the central wavelength of the narrow-band optical filter and the temperature of the LD is then controlled so that the respective voltages become equal to each other (wavelength deviation disappears), thereby making the optical output wavelength correspond to the central wavelength of the narrow-band optical filter.
In such a conventional optical transmission apparatus, the wavelength control is fixedly performed limited to a single wavelength. Therefore, in a WDM optical transmission system with a large number of wavelengths, optical devices such as LDs and optical filters and so forth are required for each wavelength. However, these optical devices suffer from problems in manufacturability and availability. Furthermore, since the wavelength control is limited to single fixed wavelengths only, this has the drawback of poor expandability on the user side. In addition, if a backup configuration for dealing with failures and so forth is considered, this also becomes extremely disadvantageous cost-wise since the number of parts is further increased.
The present invention takes into consideration the abovementioned situation, with the object of providing an optical transmission apparatus for multiple wavelengths, enabling of setting optical output wavelengths with a high degree of freedom.
In order to achieve the above object, there is provided an optical transmission apparatus for multiple wavelengths according to the present invention comprising; a light source for generating light for which the wavelength changes according to temperature, temperature control means for controlling temperature of the light source, and a wavelength control loop for detecting wavelengths of light output from the light source and feedback controlling an operation of the temperature control means based on the detected optical output wavelengths, to thereby capture the optical output wavelengths which are within a predetermined wavelength capture range into the vicinity of a target wavelength, wherein the optical transmission apparatus for multiple wavelengths further comprises a temperature control loop for detecting the temperature of the light source and feedback controlling the operation of the temperature control means in accordance with the detected temperature so that the optical output wavelengths of the light source fall within a wavelength capture range corresponding to the target wavelength of the wavelength control loop, and the wavelength control loop detects the optical output wavelengths of the light source using a wavelength detection filter having a transmission wavelength characteristic capable of setting a plurality of stable points for a wavelength capture operation, and starts the wavelength capture operation after an operation of the temperature control loop has stabilized.
With such a construction, as a preliminary operation for wavelength capture control by the wavelength control loop, a temperature control of the light source is performed by the temperature control loop so that the optical output wavelengths of the light source fall within a wavelength capture range corresponding to the target wavelength of the wavelength control loop. Then, when the operation of the temperature control loop becomes stable, the wavelength capture operation by the wavelength control loop is started. This wavelength capture operation is performed using the wavelength detection filter having a transmission wavelength characteristic capable of setting a plurality of stable points, and the optical output wavelengths of the light source are capture controlled to a stable point which corresponds to a target wavelength among the plurality of stable points.
Furthermore, in the optical transmission apparatus for multiple wavelengths, the wavelength detection filter preferably has a periodic transmission wavelength characteristic. By using such a wavelength detection filter, a wavelength deviation due to circuit errors and the like becomes small, and hence the wavelength control is accurately performed.
Moreover, in the case where the wavelength detection filter having the above mentioned periodic transmission wavelength characteristic is used, the wavelength control loop may incorporate a control polarity switching section which reverses a control direction of the capture operation in accordance with the switching of the target wavelength.
By having such a construction, the control direction of the capture operation can be selected in accordance with the set target wavelength. Therefore, the stable points of the wavelength control can be doubled.
In addition, in the abovementioned optical transmission apparatus, there may be provided control loop switching means for selectively switching either one of the temperature control loop and the wavelength control loop based on control conditions of the temperature control means.
With such a construction, the operation of the temperature control means is not controlled by a multiple loop construction of the temperature control loop and the wavelength control loop, so that a wavelength control time until the optical output wavelength becomes stable is shortened.
Furthermore, as a specific construction for the temperature control loop, this may comprise; a temperature sensor that detects the temperature of the light source and transmitting this to the temperature control means, and a target temperature setting section that sets, depending on the setting of the target wavelength, a target temperature in the temperature control means such that the optical output wavelengths of the light source fall within the wavelength capture range corresponding to the target wavelength of the wavelength control loop.
Moreover, in the abovementioned temperature control loop, it is preferable that there is provided a temperature storage section that stores the temperature of the light source detected by the temperature sensor when the optical output wavelengths of the light source have stabilized in the vicinity of the target wavelength, and the target temperature setting section sets a target temperature corresponding to the target wavelength based on storage data of the temperature storage section.
With such a construction, an influence on the wavelength control due to the occurrence of wavelength drift is reduced, enabling an even more accurate wavelength control.
Furthermore, in the abovementioned optical transmission apparatus, the construction may be such that there is provided light shut off means for shutting off light output from the light source during a period from a starting of operation by the temperature control loop until the wavelength capture operation by the wavelength control loop becomes stable.
With such a construction, since the optical output is shut off at the time of wavelength control transition, the occurrence of inter-channel crosstalk can be avoided.
In an optical transmission wavelength control method according to the present invention for detecting wavelengths of light output from a light source and feedback controlling temperature of the light source based on the detected optical output wavelengths, to thereby capture optical output wavelengths which are within a predetermined wavelength capture range into the vicinity of a target wavelength, the temperature of the light source is controlled so that the optical output wavelengths of the light source fall within a wavelength capture range corresponding to a target wavelength of the wavelength control, and thereafter, the optical output wavelengths of the light source are detected using a wavelength detection filter having a transmission wavelength characteristic capable of multiply setting stable points for a wavelength capture operation of the wavelength control, and the wavelength capture operation is started depending on the detected optical output wavelengths.